1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to an in plane switching mode liquid crystal display device and its fabrication method capable of easily controlling areas of red, green, blue and white sub-pixels arranged in a 2×2 matrix form.
2. Description of the Related Art
A twisted nematic (TN) mode liquid crystal display (LCD) device, which is commonly used as a flat panel display device achieving high image quality and low power consumption, has a problem in that its viewing angle is narrow because of refractive anisotropy of its liquid crystal molecules. This is because liquid crystal molecules are aligned parallel to a substrate in the absence of a voltage and are aligned almost perpendicularly to the substrate when a voltage is applied.
To overcome this deficiency, present research includes active development of an in plane switching (IPS) mode LCD for solving the viewing angle problem by aligning liquid crystal molecules almost parallel to a substrate.
FIG. 1 is a schematic view of a unit pixel of a general IPS mode LCD device, wherein FIG. 1A is a plan view, and FIG. 1B is a cross sectional view taken along line I-I′ of FIG. 1A.
As illustrated in FIGS. 1A and 1B, R, G and B sub-pixels construct one unit pixel, and each sub-pixel is defined by a gate line 1 and a data line 3 arranged horizontally and vertically on a transparent first substrate 10. As used hereinafter, the horizontal direction corresponds to the orientation of the gate line; and the vertical direction, which is perpendicular to the horizontal direction, corresponds to the orientation of the data line. Both the vertical and horizontal directions are parallel to the surface of the substrate. Although the n×m number of sub-pixels are defined by the ‘n’ number of gate lines 1 and the ‘m’ number of data lines 3 in an LCD device, only one set of Red, Green and Blue sub-pixels are illustrated here. As used hereinafter, the combined R, G and B sub-pixels are defined as a unit pixel.
A thin film transistor 9 of the related art is disposed at a crossing of the gate line 1 and the data line 3 defining the sub-pixel. The thin film transistor 9 includes a gate electrode 1a, a semiconductor layer 5 and source/drain electrodes 2a, 2b. 
In the sub-pixel of the related art, a common line 4 is arranged parallel to the gate line 1, and at least one pair of electrodes, a common electrode 6 and a pixel electrode 7, are arranged parallel to the data line 3. The electrodes are for switching the liquid crystal molecules. Here, the common electrode 6 and the pixel electrode 7 are respectively connected to the common line 4 and the drain electrode 2b. A passivation film 11 and a first alignment layer 12a are further applied on the first substrate 10 having the source electrode 2a and the drain electrode 2b. 
In addition, a pixel electrode line 14, which overlaps the common line 4 and is connected to the pixel electrode 7, forms a storage capacitor with the common line 4, wherein the pixel electrode line 14 and the common line 4 have a gate insulation film disposed between them.
A black matrix 21 for preventing leakage of light, and R, G and B color filters 23a-23c respectively corresponding to the sub-pixels, is formed on the second substrate 20. Additionally, a second alignment layer 12b, which along with the first alignment layer 12a determines an initial alignment state of the liquid crystal, is applied to the second substrate 20.
According to an IPS pixel structure of the related art, the liquid crystal layer 13, which is formed between the first substrate 10 and second substrate 20, transmits light according to a voltage applied between the common electrode 6 and the pixel electrode 7.
The related art IPS mode LCD device having such a structure is advantageous in that a viewing angle can be improved because a common electrode and a pixel electrode are disposed on the same plane and thus generate an in-plane electric field. However, the related art IPS mode LCD device has problems in that an aperture ratio is decreased, and its brightness is degraded because the common electrode 6 and the pixel electrode 7 are disposed in a pixel region where light would otherwise be transmitted to form an image.